1. Field of the Invention
The present invention relates generally to asynchronous, multiple-access optical communications and, more particularly, to a method of generation, and providing matched filtering for, wide-band optical carrier waveforms using code-configurable, active optical waveguiding structures.
2. Discussion of the Prior Art
The prior art has devoted considerable resources in efforts to develop communication schemes which are effective and economical. Generally speaking, optical fibers provide a communications channel with an extraordinarily large available bandwidth, which may, for example, be utilized for providing multiple access capabilities. Given such capabilities, many independent messages may be sent by many independent transmitting sources to many independent receivers along a common channel. Only two of the popular schemes proposed by the prior art to realize multiple access fiber optic communications are truly asynchronous, in the sense that data may be received by the appropriate receiver without having to synchronize (i.e. establish a time reference) with the transmitting source. These techniques are wavelength division multiplexing (WDM) and code division multiple access (CDMA).
In a WDM channel, an information carrier would be a spectrally pure (i.e. single-frequency) optical waveform. A given channel would then distinguish itself from others simply by virtue of having a different spectral center frequency than the others. Since an optical fiber is generally considered to be a linear optical waveguide, many such carrier waveforms may be superposed in a fiber without distortion, each communicating information independently of the other carriers. A receiver may then select a channel (i.e. be configured to receive only messages which are carried by a particular target frequency) simply by filtering out all wavelengths, except those in the vicinity of that frequency. Such communications are asynchronous because a receiver, tuned to an optical carrier which corresponds to the color red, for example, can receive information carried by red light without having to synchronize with the red transmitter. Thus, a given message, transmitted with an arbitrary delay by a red laser, will be received identically (except for the delay) by the red-pass filtered receiver, regardless of the delay. In contrast, synchronous schemes derive information about individual channel identity from the temporal arrangement of the received data, so that a delay (or any other uncertainty in the receiver time reference) can compromise the integrity of the channel identity.
The WDM technique can be considered an optical analog of the amplitude modulation technique, which is common in radio communications. Although WDM is simple in theory, its optical implementation can be quite challenging, particularly when a configurable (i.e. tunable) filter is desired. Despite a considerable amount of research focused on parametrically tuning the electronic resonances of optical materials to match a desired wavelength (e.g. via the electro-optic andquantum-confined Stark effects), most practical optical filtering schemes (e.g. in grating and etalon structures) achieve spectral selectivity via a tapped delay line approach, superposing many delayed copies of the received optical signal using an appropriate geometrical structure. In order to achieve an arbitrarily centered, narrow pass band, each delayed copy of the incoming light must be attenuated by an appropriate coefficient. A distributed Bragg reflector (DBR) mirror, commonly used for producing semiconductor lasers with very narrow spectral widths, is an example of this type of structure, with fixed coefficients. In attempting to extend this scheme to tunable filters, a severe practical difficulty is encountered in trying to accommodate many coefficients, which values must be variable over a continuous range. Considerable practical advantage would be gained if another type of optical carrier could be generated using strictly binary filter coefficients, signifying whether a particular delay is present or not.
Optical CDMA attempts to do this for strictly digital data using optical orthogonal codes. A CDMA carrier would be an incoherent optical waveform which is non-zero sparsely, during constant-duration (so-called chip) intervals. A matched filter for that carrier would simply consist of a delay line with equal gain taps, at delays specified by the optical orthogonal code for that chip sequence. If the matched filter receives the carrier signal for which it was designed, a large, instantaneous (correlation) peak results at the output, whose presence is ascertained via a threshold detector. If, instead, a different carrier is present, for which the filter is not matched, there will be no large peak. Such are the properties of optical orthogonal codes. Thus, communication of digital data via optical CDMA channels amounts to asynchronously observing the stream of above-threshold peak photodetector impulses at the optical filter output.
As with WDM, optical CDMA is a technique borrowed from radio communications, whose optical implementation leads to severe practical constraints. The most important limitation is that since the waveforms must be (temporally) incoherent, they cannot be encoded via a tapped delay line structure (although they may be decoded that way); they must instead be encoded by direct chip-sequence modulation of an incoherent source. However, since this chip sequence is presumably at a much higher rate than the data, this limits the data rate to be substantially lower than that for which optical modulators are capable.
A need has thus existed in the prior art for a simple, efficient, easily configurable, multiple-access optical communications scheme which allows data rates to be limited only by the speed of optical modulators.